“…51 Of the uncontrolled BnA studies, none described particular concurrent events that could influence the post-intervention vaccination coverage, except in the 2 studies that reported a vaccine shortage during the previous year 43,56 ; although in these the comparison was made with the second-to-last year, the shortage might still have biased the post-intervention vaccination coverage. In five studies ascertainment of vaccination was via self-report, 29,44,46,61,62 and in 2 the method was unclear 59,67 ; the remaining studies used objective means to ascertain vaccination. Completeness or near-completeness of outcome data could be established for 6 studies, 27,28,39,45,62,69 while 3 studies used questionnaires and had a low (<50%) response rate, raising the possibility of selection bias.…”